1) Field Of The Invention
The invention concerns an improvement in electric lamps having a lamp envelope with a phosphor coating, and more particularly, to an undercoat for increasing the luminous efficacy of such lamps.
2) Description Of The Prior Art
Low pressure mercury vapor lamps, more commonly known as fluorescent lamps, have a lamp envelope with a filling of mercury and a rare gas and in which a gas discharge is maintained during lamp operation. The radiation emitted by the gas discharge is mostly in the ultraviolet region of the spectrum, with only a small portion in the visible spectrum. The inner surface of the lamp envelope has a luminescent coating, often of a blend of phosphors, which emits visible light when impinged by the ultraviolet radiation.
Lighting accounts for approximately 20-25% of the electricity used annually in the United States. For stores, offices and warehouses, lighting may account for up to 50% of their electrical consumption. Lighting is a primary focus of The National Energy Policy Act (EPACT) of 1992, whose goal is to drastically reduce national energy consumption. This act mandates energy efficiency standards for various lamp types in terms of lamp efficacy and color rendering index. Luminous efficacy is a measure of the useful light output in relation to the energy input to the lamp, in lumens per watt (LPW). The EPACT standards for fluorescent lamps are listed in Table 1 below. Fluorescent lamps which do not meet these minimum standards cannot be sold in the United States beginning with certain dates in 1994.
______________________________________ FLUORESCENT LAMP STANDARDS NOMINAL MINI- MINIMUM LAMP MUM AVERAGE LAMP LAMP WATTAGE CRI EFFICACY (LPW) ______________________________________ 4-FOOT &gt;35 W 69 75.0 MEDIUM BI-PIN .ltoreq.35 W 45 75.0 2-FOOT &gt;35 W 69 68.0 U-BENT .ltoreq.35 W 45 64.0 8-FOOT &gt;65 W 69 80.0 SLIMLINE .ltoreq.65 W 45 80.0 8-FOOT &gt;100 W 69 80.0 HIGH OUTPUT .ltoreq.100 W 45 80.0 ______________________________________
Many fluorescent lamps on the market prior to 1994 do not meet these mandated minimum efficacy requirements. Such lamps include F96T12 and F40T12 CW (cool white), F96T12 and F40T12 Daylite, and F96T12 and F40T12 warm white. The efficacy of such lamps must be raised to the EPACT standards or they will no longer be allowed to be manufactured.
Luminous efficacy is dependent on the efficiency at which the phosphor coating converts the ultraviolet radiation from the gas discharge into visible light, on electrode losses (specifically the cathode drop), thermal losses and losses in the column gas discharge. Lamp electrodes have already been highly optimized by the lamp industry for specific applications. Losses in the column discharge are primarily a function of lamp geometry. Thus, for fluorescent lamps of a given length, diameter and lifetime, a further reduction of losses at the electrodes and in the discharge would be extremely difficult to attain.
The fluorescent layer is the single most important contributing factor in determining the luminous efficacy of fluorescent lamps. Increasing lamp efficacy through development of new or improved phosphors, however, would be very time consuming, as well as expensive. Research and development in phosphors is accomplished primarily through large numbers of experiments because results are not very predictable theoretically. Substitution of known high performing phosphors for other, lower performing ones is often not feasible either. While certain phosphors, such as the so-called narrow band and rare earth phosphors, produce high luminous efficacy and excellent color rendering in lamps intended for the high end of the market, they would be too expensive for use in lamps intended for standard or low cost applications which presently employ the relatively low cost halophosphate phosphors. Furthermore, the thickness or coating weight of each type of phosphor has already been optimized for the greatest light output. In general, if a luminescent layer is too thin, some of the U.V. radiation from the discharge will pass through the layer without being converted into visible light. If the layer becomes too thick, however, the phosphor itself absorbs and/or reflects a portion of the visible light generated in the region of the layer near the discharge.
Other types of lamps also include a luminescent layer for providing or improving their photometric parameters. These include high pressure discharge lamps, such as high pressure mercury vapor and certain metal halide lamps, which have an arc tube which at least partly emits U.V. radiation.
Accordingly, it is an object of the invention to provide a cost effective mechanism of increasing the luminous efficacy of lamps having a luminescent layer.
It is another object of the invention to increase the luminous efficacy of certain low pressure mercury vapor fluorescent lamps to meet the EPACT standards.